I'd guess you'll want to talk with somebody at Schroth USA. Their belts are quite popular with BMW enthusiasts, and yes, I know of people who use their 4- and 5-point harnesses all the time in lieu of the 3-point belts. I have seen ads that claim their products are DOT approved for street and highway use, but certainly you'd want to verify that independently.

Schroth has a toll-free number where you can inquire about local distributors: 888-536-8550 (that's from their web site, www.schroth.com). One of the favorites among bimmer fans is BMPD Design in Tyler, TX (web site www.bmpd.com).

BTW, a four-point set should suit your needs; the 5-point has a crotch strap that prevents "submarining" (sliding downward out of the belts) should you slam into something head-on at high speed.

HTH
Russ M
'82 300td
'82 300cd
(and a couple bmws)